Water is essential for human life and all known biology. Earth holds approximately 326 million cubic miles of water, roughly about 71% of the Earth’s surface. Earth sits in the perfect “Goldilocks zone”—not so far from the Sun that our water would freeze, and not too close to the Sun that water would vaporize. This has allowed stable liquid water to persist on our surface for billions of years.
How exactly Earth acquired this vast, life-sustaining inventory remains one of planetary science’s enduring mysteries. Ancient cultures viewed water as a primordial entity; representative of gods, the beginning of life and existence itself. For example, ancient Egyptians believed that the universe arose from an infinite, chaotic ocean that existed long before anything else. The creator god, called Ra in some versions, arose from this ocean on a mound of earth and brought order and life to the world. The Nile River and our oceans were believed to be extensions of this original water, and essential for fertility and rebirth. Modern science echoes this: liquid water is necessary for all known forms of life.
Understanding Earth’s water origins is crucial; it reveals the processes that deliver water or retain water on rocky worlds. Such an understanding guides our search for ice, subsurface oceans, or liquid water on other bodies. This knowledge will shape future missions hunting for resources and possible signs of habitability. Water is the linchpin for humanity’s aims to become spacefaring; without it, off-Earth bases or settlements would be a near impossibility.
THE ORIGIN OF EARTH’S WATER: INTRINSIC, EXTRATERRESTRIAL, OR BOTH?
Debates over the origin of Earth’s water have centered on whether it was native, having emerged as Earth was forming, or extraterrestrial, delivered by meteoritic bombardment. Long-standing theories posited that water could have been brought to Earth by icy planetesimals during major impact events early in the planet’s history. This would have occurred about 4.5 billion years ago, when Earth was smaller than it is now and undergoing accretion. Similarities drawn between the isotope ratios of water between the oldest known carbonaceous chondrite meteorites and meteorites from Vesta support this theory; both meteorites would have originated from the asteroid belt, which is where the frost line (the boundary of the region where ice could form in the early solar system) was located. Studies done on osmium (the densest naturally-occurring element on Earth) isotope ratios also supported this theory, suggesting that sizable amounts of water were present in the material that Earth accreted during its formation.
Meteorites offer further clues. A 2020 study published in the journal Science looked at the hydrogen content and isotopic ratios of enstatite chondrites. The belief was that since enstatite chondrites likely formed in the hot inner solar system, hydrogen would not be present. Instead, researchers studied 13 enstatite chondrites and found that they contained a shocking amount of hydrogen, suggesting that water could have been present in these meteorites at the time the
Earth formed. If Earth had been bombarded with these meteorites, it’s possible that Earth could have retained some of their hydrogen during accretion and formed water.
Similarly, a study published in 2022 in Science Advances argued that bombardment of the Mars following its accretion by chondritic meteorites carrying water may have provided an exotic water inventory. Martin Bizzarro, one of the co-authors of the study, stated that “the amount of water delivered to Mars was enough to make a planet-wide ocean 300 meters thick.” Analysis of the isotopes in the meteorites by the researchers showed that the chondrites likely came from the outer solar system; when studied, meteorites originating from this region have been shown to contain water and carbon, while meteorites from the inner solar system are dryer. Even if these chondrites only contained 10% water, Bizzarro and his colleagues suggest that they could have transported liquid equal to more than 300 meters GEL, and possibly organic molecules, on Mars.
Water has even been detected in Martian meteorites; in 2013, researchers found that a Northwest Africa 7034 (often called “Black Beauty” after its particularly rich, dark fusion crust and matrix) sample contained approximately 6,000 parts per million of water, the highest content of any Martian meteorite found to date. NWA 7034 is also the oldest Martian meteorite found to date, at 4.4 billion years old. The age of the stone and its high water content provide clues about how just low long water has been present on the Red Planet; researchers believe water could have been present since the planet’s formation. Martian meteorites in the nakhlite class have been found to contain iddingsite, which is produced when olivine interacts with liquid water.
Northwest Africa 7034, nicknamed “Black Beauty”It’s also been suggested that, in the case of Earth and Mars, bombardment by meteorites carrying water occurred in addition to mantle outgassing, which would have occurred during formation. Volatile compounds like water vapor, carbon dioxide, and sulfur would have been released from the planet’s interior to the surface, likely due to volcanic activity. A similar process would have happened on Earth, early in our history. William W. Rubey’s seminal 1951 paper titled “Geological History of Sea Water: An Attempt to State the Problem” was hugely influential in outlining this idea.
A 2025 paper published in Nature further explored the notion that Earth’s water came from within itself during formation. Miozzi, et al. performed experiments to test the notion that the reaction between atmospheric hydrogen and an underlaying magma ocean could produce copious amounts of water. Laser heating diamond anvil cell experiments supported the hypothesis that “the typical processes attending planet formation will result in substantial water production.”
Another 2025 study published in Icarus looked to meteorites to substantiate claims such claims about the origins of water on Earth. Researchers at the University of Oxford studied an enstatite chondrite called LAR 12252, found in Antarctica, which is thought to be similar in composition to Earth early in its formation about 4.5 billion years ago. They found a surprising hydrogen source which would have been instrumental for water molecules to form. Further, they found that the hydrogen present in the meteoritic material was native and not from contamination, suggesting that Earth may have had the hydrogen it needed to form water as it was accreting.
THE SEARCH FOR WATER CONTINUES
Our planet remains the only known body with stable bodies of liquid water on its surface. However, we now know that water may have once covered large parts of Mars and Venus, and water ice exists in the permanently shadowed polar regions on the Moon, and perhaps even on sunlit surfaces. Jupiter’s moon Europa may contain a global subsurface of liquid saltwater. The search for water on other planetary bodies is critical in our aim to become a spacefaring
species. How to obtain or produce water is critical to designing any human settlement off-Earth; as such, several upcoming spaceflight missions are focused on investigating water on other planetary bodies like the Moon, asteroids, and Jupiter’s moons.
Mars’s water has been a focus of scientific investigation since the 1800s. When viewed through a telescope, white polar caps and clouds can be observed on its surface. As such, early observers assumed that these features indicated the presence of water on the Red Planet. The Italian astronomer Giovanni Schiaparelli—after which the 285-mile-wide Schiaparelli crater on Mars is named—was the first to produce a detailed map of Mars. He noted that there were long, straight lines on the Martian surface, and he dubbed them canali, or channels. Percival Lowell, the founder of the Lowell Observatory in Flagstaff, Arizona, believed that these channels were created by Mars’ inhabitants to tap the polar ice caps for their water.
Illustration of Mars and its canals by Percival Lowell.he canali were eventually disproven as an optical illusion, and Lowell’s ideas were rejected. The scientific establishment at the time presented a different view of Mars, as a desolate, cold, and barren planet. Water vapor, however, was detected on Mars in 1963 at Mount Wilson Observatory using ground-based spectroscopy. The Mariner 9 mission, launched in 1971, produced thousands of the images of the Martian surface, capturing volcanoes, valleys, and most interestingly, dried riverbeds.
Robotic probes have revealed even more about Mars’s history, and raised even more questions. The Viking orbiters and landers, which operated between 1976 and 1982, photographed outflow channels on the Martian surface which suggested that floods of water once cut grooves into bedrock and carved deep valleys. Evidence of possible rain was found, along with chemical analysis of the Martian surface suggesting that it could have submerged in water in the past. In 1998, the Mars Global Surveyor orbiter found that the northern polar ice cap had a topography consistent with water ice. The Thermal Emission Spectrometer instrument, which aimed to determine the mineral composition of Mars’s surface, found that it contained olivine, which suggested that at least parts of Mars had been dry for a long time.
The Pathfinder lander (1997-1998) also recorded the Martian atmosphere, and determined that it was likely too low to permit liquid water to exist on Mars’s surface, though more evidence was found that was consistent with water being present there in the past. These included rocks and pebbles that looked rounded, as if they had been tumbled in a stream. Other rocks were observed to be positioned in such a way that they had been pushed together by flowing water. The InSight lander, which touched down on Martian soil in 2018, took seismic measurements which researchers later analyzed and found that a reservoir of liquid water could exist deep beneath the planet’s surface, 6.2-12.4 miles (10-20 kilometers) under the crust.
If there was a way to tap these possible water resources, the implications for human settlements on Mars are huge, not only for consumption but also fuel production. Via the Sabatier process, hydrogen (H2) can be separated from oxygen in water; the hydrogen is then combined with atmospheric carbon dioxide to create methane rocket fuel. This would decrease our dependence on Earth for supplies and bolster our ability to further explore our solar system.
Water also remains a target for exploration for future crewed and robotic missions to the Moon. China’s Chang’e-7 mission, for example, will target the south pole of the Moon and deploy some of their most sophisticated spacecraft (some built with international cooperation) to seek out water ice in the Moon’s permanently shadowed regions. If water is found there, then scientists will assess the potential for in-situ resource utilization for a possible human presence on the Moon, like the International Lunar Research Station initiative lead by China and Russia. Chang’e 7’s hardware includes an orbiter, a lander, a rover, and a hopper. Chang’e 7 is scheduled to launch in the back half of 2026.
NASA’s Volatiles Investigating Polar Exploration Rover, or VIPER, mission is slated for 2027 and has similar aims. The mission was cancelled in 2024, with the agency citing “overall Science Mission Directorate funding constraints, future budget risks, and lander delays.” However, Blue Origin was selected in 2025 to carry VIPER to the Moon aboard its Blue Moon MK1 lander. The robot will land in the Moon’s south pole, one of the coldest regions in the solar system, and embark on a 100-day mission to map the area and search for water ice.
Crewed missions to Mars will prioritize searches for water for sustainability. Earth’s water story, whether it was delivered from space or born inside, teaches us what conditions allow a world to hold liquid water long enough for life to emerge and thrive. Solving this mystery would enable humanity to chart the path to finding and using water beyond our planet, turning science fiction into the next chapter of human exploration.
